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Lasers from Nanowires?

Our computers nowadays continue to shrink while they grow faster and stronger in terms of computing power. People require faster laptops, faster tablets and smartphones so that they can surf the internet, call friends and family, or, as some do, they simply wish to compute and run programs which are increasingly more complicated and require better, more advanced machines. Whatever the reason, people want and need something faster and smaller than ever before.

Plasmonic laser geometry and light emission (2)

The basis of computers are integrated circuits containing many components which are made tiny in order to fit in small dimensions and work perfectly. Some researchers are, however, still pushing the boundaries of what a computing machine can do and how quickly it can get the job done. One such idea is abandoning the use of electronic circuits which have copper connections. A very interesting project that is now gaining speed is an integrated circuit with optical interconnects. This would mean that instead of having conducting connections between parts of a chip there would be a waveguide which would funnel light from a source and use it in place of electricity. This could mean creating transistors and even whole computers which use light instead of electricity for transferring information. The idea behind this is that the optical interconnects would make it possible for much larger volumes of data to be transported through the circuits. This means that one could make a computer that has a 100 GHz processor instead of the 1 or 2 GHz that we have now. Small optical circuits require small sources of light. The ideal candidate for this would be a laser which is on the nano-scale or a nanolaser. There are many different kinds of nanolasers but two of these are the most interesting: nanowire lasers and plasmonic lasers.

Nanowire lasers are made from semiconductor cylinders which are several micrometers in length and several hundred nanometers in diameter (1). When viewed through an electron microscope they are seen as small needles placed either vertically on a substrate or horizontally. In order to make these nanowires into lasers one needs a pump beam or a larger source of light such as a conventional laser. When the light from this pump is shone on the nanowires they begin to emit light themselves from the end facets of the cylinders (1). The only difference now is that the light is much more focused and has a much smaller spot size which is what is required for a circuit. The advantage of this type of nanolaser is that it can easily be integrated onto a chip and additionally this structure exhibits small losses when emitting light (1). On the other hand the light from this laser is diffraction limited. This is a fundamental restriction on the size of the laser compared to the wavelength of light it produces. This means that a source of light much smaller than the wavelength that it produces would have a diffracted beam or simply put, the light would not be focused anymore. The next type of lasers, however, manage to work around this limit.

Plasmonic lasers are similar in the sense that they also require nanowires to function but they run on a different mechanism. For this type of nanolaser, nanowires are arranged horizontally on a metal surface. They are not, however, touching the surface but are in fact separated by a small insulating layer (2). In this way when a pump beam is shone on the nanowire the system begins to emit light from the end of the cylinder as well. The light, however, does not come from the nanowire facet itself but from a place below, closer to the metal surface as can be seen in the figure above. This type of laser is unique in that it stores most of its energy in surface plasmons. This is a coherent excitation of the electrons on the surface of a metal. Through this coupling of the plasmons and the nanowire one can in fact create a laser smaller than allowed by the diffraction limit (2). The advantage here would be that one could create much smaller lasers and therefore would be able to make a much smaller source of light for a compact integrated circuit than any other method. Disadvantages, however, are that there are large losses of energy within the metal and that the research is still in the early phases of development and more work is required in order to easily integrate it onto a chip.

Nanolasers and especially plasmonic lasers still require quite a lot of work in order to become feasible for applications. There is also research on how exactly to create the nanowires in these lasers and how to improve upon them to make them efficient and usable by industry. The idea of optical circuits is also far from completed and some parts of the research are still in their infancy but judging by the number of people working on such projects we should not have to wait long.

Panaiot Zotev did his undergraduate studies in Physics at the University of Virginia in the United States. He was later accepted at TU Munich for a master’s degree in Applied and Engineering Physics. Some of his interests include semiconductor physics, nano-lasers, and their applications in optical circuits used for computing.

Deividas Sabonis did his undergraduate studies in Physics jointly in Vilnius University and University of Copenhagen, Niels Bohr Institute. He was awarded a graduate studies fellowship by the German Academic Exchange Service or DAAD for the period of 2014-2016 at TU Munich. Main interests: quantum optics, light matter interaction.